I would suggest getting the Mechanism drive belts changed, as these stretch and and become brittle over time.
It would be an idea to also regrease the gears, this can sometimes prevent the laser assembly locking up into position fully.
Also check the ribbon connections and control cables to the laser and drawer mech unit, and main board are seated fully.
I have known these come loose, causing intermittent problems.
Failing that, the complete cd drive mech unit can be replaced, which includes the laser and drawer mech.
Cost I would estimate around £50.
There may be an electronic fault, capacitors in the power supply are always favorite.
If you not sure send the unit in for service or contact a Denon service centre for more information.
Hope this helps.